U.S. Pat. No. 5,110,478 describes the direct synthesis of zeolite membranes. The membranes produced in accordance with the teachings of U.S. Pat. No. 5,110,478 were discussed in "Synthesis and Characterization of a Pure Zeolite Membrane," J. G. Tsikoyiannis and W. Haag, Zeolites (VOl. 12, p. 126., 1992). Such membranes are free standing and are not affixed or attached as layers to any supports. Furthermore, the membranes have a gradient of crystal sizes across the thickness of the membrane. Such a gradient precludes growth of a thin membrane with a minimum number of nonselective permeation paths.
Zeolite membranes have also been grown on supports. See e.g. "High temperature stainless steel supported zeolite (MFI) membranes: Preparation, Module, Construction and Permeation Experiments," E. R. Geus, H. vanBekkum, J. A. Moulyin, Microporous Materials, Vol. 1, p. 137, 1993; Netherlands Patent Application 91011048; European Patent Application 91309239.1 and U.S. Pat. No. 4,099,692.
All of the above prepared membranes have nonuniform sized zeolite crystals and are noncontinuous, exhibiting many voids. Obtaining functional zeolite membranes from low alkaline synthesis routes is difficult because the heterogeneous crystals in the membrane require an enormous membrane thickness to seal pinholes and void structures which lower the membrane selectivity.